Am looking to install Anderson double hung new construction windows into an existing opening, replacing older single hung windows (that have old wood window sills and exterior trim that I want to eliminate/replace).

Most of the research/guides/videos I've viewed show in detail the install procedure when the siding hasn't yet been installed, i.e. showing Tyvek over framing.

1. Once I have the window in I need to flash it. Most sites recommend installing 4" flashing around the entire window, however the gap over existing rough opening to siding is 2 1/4". Is this enough of an overlap. If not what's recommended ?

2. Some sites say not to flash below the sill. Some say you should. Which is right? (I know not to caulk at the bottom).

3. I would like to install a sill pan. Am I right in thinking if the sill pan has a back dam then I will need to use some kind of non-organic flat shims to raise the window above the height of the back edge ?

Any other advise on this type of install would be really appreciated ...